Ultrahard nanotwinned cubic boron nitride

Area of Science:

• Materials Science
• Nanotechnology
• Superhard Materials

Background:

• Cubic boron nitride (cBN) is a superhard material with significant industrial applications.
• Nanostructuring cBN enhances hardness via the Hall-Petch effect, where smaller grain sizes increase hardness.
• Conventional synthesis methods produce polycrystalline cBN with grain sizes around 14 nm.

Purpose of the Study:

• To synthesize and characterize a novel nanostructured cBN material with enhanced mechanical properties.
• To investigate the relationship between nanostructure, specifically twin domain thickness, and cBN hardness.
• To explore the potential of nanotwinned cBN for demanding industrial applications.

Main Methods:

• Synthesis of nanotwinned cBN using specially prepared BN precursor nanoparticles with nested structures and puckered layers.
• Characterization of the nanostructure, focusing on twin domain thickness (average ~3.8 nm).
• Evaluation of mechanical properties, including Vickers hardness, oxidation temperature, and fracture toughness.

Main Results:

• Formation of optically transparent nanotwinned cBN bulk samples.
• Achieved Vickers hardness exceeding 100 GPa, surpassing synthetic diamond.
• Demonstrated high oxidation temperature (~1,294°C) and fracture toughness (>12 MPa·m^(1/2)).
• Observed continuous hardening with decreasing twin thickness, deviating from the typical Hall-Petch or reverse Hall-Petch effects.

Conclusions:

• Nanotwinned cBN synthesized from nested precursor nanoparticles exhibits superior hardness and toughness.
• The observed hardening behavior suggests a new mechanism related to ultra-fine twin domains.
• This advanced cBN material holds promise for applications requiring extreme hardness and durability.